You can drag the picture to your Desktop. Then drag from the Desktop to the PowerPoint slide.
You can turn your cursor into a camera by holding down Apple + Shift + 4. Scroll over the image you want. The image is then saved to your Desktop. Drag the image from the Desktop to the PowerPoint Slide.
How do I get a image from a webpage?
How do I copy, cut, and paste?
How do I move slides around?